Grass grows through the floor of my building
I am guessing you are using a hair particle system for your grass. If so: Use a vertex group and weight painting to control placement of grass (and its length, if you want; see : Increase Intensity of Vertex Group ). 

In edit mode, subdivide the mesh on which the grass is placed, select the mesh squares outside of the house and assign them to a vertex group. You can control the placement even more if you then switch to weight paint mode.



This use the vertex group control in your particle system to place the grass only in the vert'ed areas.



You'll end up with something like this, but prettier):



Also related, particularly about finer placement of hairs:

How do I make hair particles appear on top of the emitter and not through it?

Hair renders from regions outside vertex group